Many satellite communications systems have Adaptive Coding and Modulation (ACM) that aim to maximize the throughput of a forward link (i.e. for transmission to a user terminal (UT)). An example system is disclosed in US2014/0056335 A1.
FIG. 1 depicts a satellite communication system comprising a ground station 1 that transmits uplink signals UL to a satellite 2, which transmits corresponding downlink signals DL in one or more beams. A plurality of UTs (User Terminals) 6 in a beam 5 are served by two forward carriers FC1 and FC2. The forward carriers FC1, FC2 are shared between multiple user terminals 6. In a typical satellite communication network both the forward carriers FC1, FC2 will operate with a fixed EIRP (Effective Isotropic Radiated Power).
Consider an example where a UT 6 is making a voice over internet-protocol (VOIP), and is in an area where there is excellent signal strength. In a conventional ACM method the UT 6 will report its link conditions and the network will adapt the code rate and modulation so that the user can achieve maximum data rate. However, the UT 6 only requires a sufficient data rate to make a VOIP call, whereas the maximum data rate may only be required if the UT 6 is streaming real time data. Hence, the conventional approach to ACM may not give the optimum overall system performance.